In recent years, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a battery module having a plurality of battery cells electrically connected to each other because high output and large capacity are necessary for the middle or large-sized devices.
Preferably, the battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell (a unit cell) of the battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing cost of the pouch-shaped battery is low, and it is easy to modify the shape of the pouch-shaped battery.
FIG. 1 is a perspective view showing a pouch-shaped battery cell, which is included in a conventional battery module, and FIG. 2 is an exploded perspective view of FIG. 1.
Referring to FIGS. 1 and 2, the pouch-shaped battery cell is a plate-shaped battery cell 10 having electrode leads (a positive electrode lead 11 and a negative electrode lead 12) formed at one end thereof. The pouch-shaped battery cell is configured to have a structure in which an electrode assembly 30, including positive electrodes, negative electrodes, and separators respectively disposed between the positive electrodes and the negative electrodes, is mounted in a pouch-shaped battery cell case 20 in a state of being sealed by a sealed portion formed by thermal bonding such that two electrode leads 11 and 12, welded to positive electrode and negative electrode tabs 31 and 32 of the electrode assembly 30 such that the electrode leads 11 and 12 are electrically connected to the positive electrode and negative electrode tabs 31 and 32, are exposed outward from the pouch-shaped battery cell case 20.
FIG. 3 is a typical view showing a structure in which pouch-shaped battery cells are electrically connected to one another.
Referring to FIG. 3 together with FIG. 2, electrode leads 11a, 11b, and 11c of three adjacent pouch-shaped battery cells 10a, 10b, and 10c are coupled to one another such that the electrode leads 11a, 11b, and 11c are electrically connected to one another. The electrode leads 11a and 11c of the battery cells 10a and 10c, which protrude outward from battery cell cases of the battery cells 10a and 10c, are bent such that the electrode leads 11a and 11c are connected to the electrode lead 11b of the battery cell 10b adjacent thereto. The electrode leads 11a, 11b, and 11c are electrically and physically coupled to one another by ultrasonic welding performed in a state in which the electrode leads 11a, 11b, and 11c overlap with each other.
When ultrasonic welding is performed using high energy, strength of welded portion between the electrode tabs 31 and 32 and the electrode leads 11 and 12 is reduced due to vibration generated during ultrasonic welding with the result that the electrode tabs 31 and 32 and the electrode leads 11 and 12 of the battery cell 10 may be broken or short-circuited. Besides, the vibration generated during ultrasonic welding may be transferred to other portions of the battery cell 10 with the result that the battery cell 10 may be electrically damaged.
In addition, when a battery module is assembled or when the battery module is mounted in a device, the coupled portions between the electrode tabs and the electrode leads may be damaged due to external force with the result that the safety of the battery module may be reduced. For example, when the battery module is transported or when the battery module is mounted in a device, such as an electric vehicle, the coupled portions between the electrode tabs and the electrode leads may be damaged due to external force or vibration applied to the battery module.
Therefore, there is a high necessity for a battery cell that is capable of preventing the breakage of, short circuit of, or damage to a coupled portion of electrode leads due to external force or vibration generated during ultrasonic welding.